Diorite is a coarse-grained, intrusive igneous rock that contains a mixture of feldspar, pyroxene, hornblende, and sometimes quartz.
One of the most familiar specimens is called “salt and pepper” It is given that name because of its appearance. It is produced when White Plagioclase Contrasting with Black Hornblende and Biotite combine below earth’s surface.
Diorite is the name used for a group of coarse-grained igneous rocks with a composition between that of granite and basalt. It usually occurs as large impingements, and sills within continental crust. These often form above a convergent plate boundary where an oceanic plate subducts beneath a continental plate. Partial melting of the oceanic plate produces a basaltic magma that rises and obtrudes the granitic rock of the continental plate. There, the basaltic magma mixes with granitic magmas or melts granitic rock as it ascends through the continental plate. This produces a melt that is intermediate in composition between basalt and granite. Diorite forms if this type of melt crystallizes below the surface.
Diorite and Andesite
Diorite and andesite are similar rocks. They have the same mineral composition and occur in the same geographic areas. The differences are in their grain sizes and their rates of cooling. Diorite crystallized slowly within the Earth. That slow cooling produced a coarse grain size. Andesite forms when a similar magma crystallizes quickly at Earth’s surface. That rapid cooling produces a rock with small crystals.
Uses of Diorite
In areas where diorite occurs near the surface, it is sometimes mined for use as a crushed stone. It has a durability that compares favorably to granite and trap rock. It is used as a base material in the construction of roads, buildings, and parking areas. It is also used as a drainage stone and for erosion control.
In the dimension stone industry, diorite is often cut into facing stone, tile, ashlars, blocking, pavers, curbing, and a variety of dimension stone products. These are used as construction stone, or polished and used as architectural stone. Diorite was used as a structural stone by the Inca and Mayan civilizations of South America and by many ancient civilizations in the Middle East.
In the dimension stone industry, diorite is sold as a “granite.” The dimension stone industry uses the name “granite” for any rock with visible, interlocking grains of feldspar. This simplifies discussions with customers who do not know how to identify igneous and metamorphic rocks
Diorite might appear in a polished countertop, facing stone, or floor tile. It would probably be marketed as “white granite” at a cabinet shop or building supply store. A Neolithic ax made of diorite that was found in the surroundings of Reims, France. It is in the Alexis Damour Collection at the Museum of Toulouse.
Diorite Sculptures: There is a famous sculpture of Gudea, a Mesopotamian ruler, made in about 2090 BC. It is about 19 inches tall and is currently displayed at the Metropolitan Museum of Art. Moreover, there was a vase made in ancient Egypt from diorite with spectacular feldspar phenocrysts. It is in the collection of the Field Museum.